Ranking system regarding Mastery

I really don't understand how it grades you. Especially for supports, like it makes zero sense.

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2536802304/39816033?tab=overview

I had 73% kill participation. Zero deaths. I was warding and doing my job as a Support and I get a fucking measly A+? What is this. Where as my friend gets a S+ as Caitlyn for doing what? Not dying and getting a few kills.

tl;dr I want a more clear understanding why I am getting the rank I do. There is no insight on how to improve my rating.